The present invention relates generally to vacuum switching tubes for low-voltage and medium-voltage switches, and more particularly to vacuum contactors having a switching chamber and a first contact piece fixed in place in the switching chamber, as well as a movable current conducting pin with a second contact piece and a ring-shaped insulator.
Many various versions of vacuum switches are presently available in the industry. The purpose of such switches is to allow current flow by closing an open switch, to conduct current in a closed state of the switch, and to interrupt current flow by opening the switch. In the closed state of the switch, the two contact pieces touch mechanically at the contact surfaces, and thus allow an electrically conductive connection, i.e., current can flow. In contrast, in the open state of the switch, the two contact pieces are mechanically separated, so that the insulation medium of the vacuum does not permit any current flow between the contact pieces.
When the switch is mechanically opened under a load, i.e. when there is current flow and the switch is opened, a metal vapor arc occurs due to local overheating at the contact site, producing a conductive connection between the contacts. The switch only opens electrically in the vicinity of the current zero crossing at the end of a current half-wave. If the metal vapor cools rapidly enough during a current zero crossing and condenses at cool regions of the switch, a sufficiently conductive medium (plasma) no longer exists. The recurrent voltage is present at the two contact pieces and thus also at the insulator, if the switch has opened successfully.
The latter insulator is ring-shaped, due to the structure of vacuum switches, which are usually in the form of hollow cylinders, and must have high insulation capacity both in the interior region of the tube and in the exterior region, until the end of the useful lifetime of the switch. A significant design objective in the implementation of a vacuum switching tube consists of designing enough vapor surfaces for cooling and condensation of the metal vapor, but at the same time preventing condensation on the vacuum side of the insulator in those regions which are necessary for maintaining the voltage withstand capability.
In the case of known vacuum switches, the region of the insulator required to achieve the necessary voltage withstand capability, in a vacuum is frequently protected against evaporation by one or more metallic shields. For example, in the vacuum switch disclosed in DE-B-38 40 192, the center part of the ring-shaped insulator is protected by a special vapor shield in the shape of a hollow cylinder, affixed in the interior region of the switch. Also, a vacuum switch for the low-voltage range, to be used as a low-voltage contactor, is disclosed in EP-B-0 149 061. The vacuum switch in this configuration has an insulator, which on the vacuum side relative to the contact pieces, is covered by a shield structured as a concentric hollow cylinder. The axial length of the shield is at least 1.5 times the length of the ring-shaped insulator. The ring-shaped insulator of ceramic material is connected on one side with a metal bellows extending around the vacuum switching tube and surrounding the movable current conducting rod. The outside circumference of the shielding cylinder has a radial distance between 0.5 and 3 mm. from the inside circumference of the ring-shaped insulator and from the inside circumference of the bellows.
The structure and production of such hollow cylinder vapor shields involve significant effort and expense. It has already been proposed that to avoid separate vapor shields, the wall thickness of the insulator be increased, and the insulator be provided with indentations on the side facing away from the switch contacts, so that a part of the insulator itself takes over the function of the vapor shield. Such a solution, however, requires the use of more material, and greater production effort and expenditure for the insulator, resulting in corresponding higher costs.
Furthermore, a vacuum housing for circuit breakers capable of functioning without vapor shields is known from DE-A-37 09 585. For this purpose, the movable current conducting rod is surrounded by a ceramic element, which causes a narrowing of the passage cross-section between the switching chamber in which the arc occurs when the breaker is opened, and a folded bellows, designated as a corrugated tube, located behind the ceramic element. The ceramic element shields the folded bellows from the arc. However, this reference discloses nothing concerning the insulation strength.
The present invention is directed to the problem of developing vacuum switching tubes for low-voltage and medium-voltage applications, which do not necessarily require vapor shields to maintain the insulation capacity when the switching segment is open, and which, in particular, do not have any steps or undercuts on the inside of the insulator that would complicate the production of these tubes.